The invention relates generally to solid state welding technology, and more particularly to friction stir welding.
Increasing the output and efficiency of turbo-machinery such as gas turbine engines requires optimization of materials that balance high temperature strength, creep and fatigue resistance, oxidation and corrosion resistance, as well as structural stability, among others. In many cases, the alloying content requirements of these materials have dictated a powder processing approach to prevent material segregation. When joining these, as well as many conventionally cast materials, it is often advantageous to remain below the melting temperature, thereby eliminating issues commonly observed in traditional fusion welding processes, such as solidification induced cracking and porosity, weld zone material segregation, and the formation of a rapidly solidified cast microstructure.
Solid state welding or joining processes have been developed as a way of addressing these issues. One of the more successfully employed techniques is friction stir welding, which can be used to join similar or dissimilar metals and alloys, thermoplastics, or other materials. The solid-state nature of this technique addresses the above mentioned issues associated with other more conventional joining techniques, enabling the joining of materials otherwise considered difficult or impossible to weld.
In a typical friction stir welding system, a rotating, often cylindrical, consumable or non-consumable pin tool may be plunged into a rigidly clamped workpiece at a location containing a linear or non-linear joint to be welded. Frictional heating locally plasticizes the workpiece, enabling material transfer across the joint interface through a forging and/or extrusion action about the rotating pin tool. Ideally, workpiece temperatures remain below the melting temperature of the material throughout the duration of the weld. In many material systems, precise through-thickness control of in-situ weld metal heating and cooling rates is also critical to the quality of the weld. Improved control over in-situ pin tool and workpiece temperatures can also prevent bonding between the workpiece and the backplate, undesirable workpiece material structure, and destruction of the backplate components.